Characteristics and genesis analysis of middle and lower Ordovician dolomite in the southern margin of the Ordos Basin, China

The Middle-Lower Ordovician formations in the southern margin of the Ordos Basin host multiple sets of dolomites, which readily form high-quality reservoirs with promising exploration prospects. Understanding the geochemical characteristics and analyzing the genesis of these dolomites in this region is crucial. Based on drilling, logging, core observation, and thin section identification, representative dolomite samples from the Xiweikou, Jueshangou, and Caojiagou profiles were selected for comprehensive geochemical analysis. Combined with the petrological characteristics and cathodoluminescence features of the dolomites, their characteristics and genesis were analyzed. The results indicate that the average 87 Sr/ 86 Sr ratio of 11 dolomite samples in this area is 0.710842. The rare earth element distribution pattern shows an enrichment of light rare earth elements (LREE) and a depletion of heavy rare earth elements (HREE). The micritic and finely crystalline dolomite crystals are euhedral to subhedral, with low ordering, high Na, K, Sr, and low Fe, Mn content. Under cathodoluminescence, they emit rose-red to dark-red light, indicating a product of penecontemporaneous dolomitization. The sandy crystalline dolomites (fine, medium, and coarse crystalline) generally have anhedral to subhedral crystal forms, high ordering, low Na, K, Sr, and high Fe, Mn content. Under cathodoluminescence, they emit dark-red or no light, indicating a product of burial diagenesis. • Three main dolomite types—micritic, finely crystalline, and sandy crystalline—were identified in the Middle-Lower Ordovician strata of the Ordos Basin. • Significant differences in dolomite ordering, geochemical composition, and cathodoluminescence characteristics reflect varied diagenetic environments. • Two genetic models for dolomite formation are proposed: penecontemporaneous dolomitization in shallow, high-salinity environments, and burial dolomitization in deep settings. • The study's findings on dolomite characteristics and genesis provide valuable insights for predicting reservoir quality in the Ordos Basin.